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The human body is primarily made of water, and this water must be retained. The skin is constantly shedding dead keratinocytes (skin cells) from its surface while deeper living keratinocytes divide to replenish them. This layer of dead cells also has interspersed natural oils which combine with the dead cells to create a barrier to water. Keratinocytes also secrete keratin, a structural protein, to assist in the formation of this impenetrable barrier. At times, the body must quickly shed heat, and sweat glands act as pores through the skin surface allowing internal, higher temperature, water and salt to leave in the form of sweat.

A. dead keratinocytes and oil create a barrier to water penetration. Correct.

B. false, water easily crosses the skin by passive diffusion through sweat glands. Incorrect, sweat glands act as small pores through the skin with open channels. Sweat does not pass through the skin by passive diffusion, it is secreted by cells in sweat glands.

C. skin has a low osmolarity, thereby preventing influx of external water. Incorrect, Water does not pass through skin because of the barrier created by hydrophobic oils and dead cells. The barrier has nothing to do with low osmotic pressure.

D. skin cells have temperature-gated membrane channels that allow sweat to leave only at increased body temperatures. Incorrect, Skin cells do not have temperature-gated channels, and the cells in the outermost skin layer are dead.